Power take off from an open-centre hydroelectric turbine is most conveniently arranged by means of an electrical generator of rim construction located at the outer edge of the rotor and inner edge of the stator. In such cases the generator is a synchronous machine of high pole number. The field system may use electrical coils supplied with current or a set of permanent magnets to provide the magneto motive force required to drive magnetic flux through the magnetic circuit. This arrangement gives a large diameter open space in the centre of the stator that accommodates the rotor. The rim generator operates at the same rotational speed as the turbine and requires no gearing.
Directly-Driven (i.e. gearless) Generators with Permanent-Magnet field excitation (DDPMGs) offer the simplest and potentially most reliable and cost-effective form of power take-off device for renewable energy systems.
The majority of DDPMG designs employ a magnetic circuit created by a set of magnets on the rotor forming a radial magnetic field within a narrow gap separating the rotor and stator. The stator is usually of essentially conventional construction and includes an electrical circuit based on insulated coils intertwined within slots in the bore of a cylindrical laminated iron stator and resembles the stator of an induction or synchronous machine. The magnetic circuit is coupled to the electrical circuit by virtue of the location of the rotor within the stator. The magnetic circuit normally includes ferromagnetic sections made of iron or steel to provide a path of low reluctance for the passage of magnetic flux. Such sections are usually provided within both the rotor and stator.
The magnetic field established by the field system passes across the gap that separates the rotor and stator. Relative movement of the rotor, and therefore magnetic field, with respect to the stator, and therefore the stator coils of the electric circuit, causes an electromotive force (EMF) to be induced in the coils. However, the flux linkage with any other circuit within the stator also undergoes changing flux linkage and cmf is induced. In order to avoid unwanted current flowing in the magnetic iron or steel of the stator, which results in power loss, it is usual to construct the core of the stator, onto which the coils are wound, from thin sheets of magnetic iron or steel separated by electrical insulation. The sheets are called laminations and are cut to shape by a punching process. Insulation is usually provided by a thin coating to one or both sides of the sheet from which the lamination is punched. The armature coils are usually attached to the laminated magnetic core by forming slots during the punching process. The coils have to be inserted and secured in the slots and this process stresses the winding insulation, and often means that thicker insulation is needed than would be required simply for electrical isolation.
Where small numbers of machines are required the cost of producing the die for punching the laminations can be an important component of the final machine cost and the time taken to make the special die can delay construction. In addition, the material removed from the centre of the punched lamination is wasted, which represents a considerable cost.
In hyrdoelectric applications it is desirable to use a large-diameter machine, which can lead to improved efficiency and reduced use of electromagnetically-active material. However, for machines of large diameter it is necessary to construct the laminations as a set of arc segments, because it is not possible to obtain magnetic steel sheet in sufficient size for producing a complete ring. The arcs must be housed in a supporting structure. This adds significant cost to the machine.
This laminated, slotted stator arrangement results in the formation of teeth which project towards the rotor, thus resulting in the rotor being attracted toward a preferred angular position, an effect known as “cogging”, which then requires considerable drive torque to start the rotor. Furthermore, the large radial attraction force between rotor and stator requires a massive stiff supporting structure.
The present invention has therefore been developed with a view to mitigating the above mentioned problems.